The following discussion of the background of the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge in any jurisdiction as at the priority date of the application.
HF SSB frequency hopping networks change operating frequencies several times per second. In order to ensure that communication is maintained between senders and receivers, all stations must be synchronised so that they change to the same frequency at the same time (a process commonly referred to as “hopping”).
One method of synchronising communications between land-based senders and land-based receivers in a HF SSB frequency hopping network is to designate a sender as a master station. The master station sends synchronisation data to the receivers via several frequencies within the hopping channel. The receivers process the synchronisation data to determine the time of the next change and the frequency range of the next hopping channel.
The problem with this method of synchronisation is that if the synchronisation data is not received by a receiver or an incomplete set of synchronisation data is received, the receiver is most likely to lose communication with the master station (or fail to establish communication if the missed or incomplete synchronisation data has been sent in order to initiate communication between senders and receivers). The receiver must then wait until the master station again transmits synchronisation data on frequencies within the hopping channel used prior to loss of communication before it can re-establish communication. This delay before re-establishing communication can be a significant period of time and may result in degradation or loss of communication at a critical time.
This problem is further exacerbated when it is considered that the synchronisation data may be lost due to reasons such as signal path propagation failure and local noise or other interference.
Another method of synchronising communications between land-based senders and land-based receivers utilises both frequency scanning systems and selective calling systems. Frequency scanning systems that also utilise selective calling systems operate as follows.
All stations in the communications network scan, and receive, frequencies throughout the High Frequency spectrum that they are allocated to use. A station wishing to call another station selects a frequency and sends a selective call signal addressed to the station it wishes to call. If the recipient station hears the calling station it sends a return signal to the calling station indicating the signal quality of the selected frequency. If the calling station does not receive a return signal, or the signal quality described by the return signal is not sufficient for the proposed communication, the calling station selects another frequency and repeats the process. This continues until a suitable frequency is found.
The problem with this synchronisation method is that there may be a significant delay before an acceptable return signal is received from the recipient station and this delay may result in a loss of communication at a critical time.
Another method of synchronising communications between land-based senders and land-based receivers utilises both frequency scanning systems and Automatic Link Establishment (“ALE”) (described in FED-STD-1045). Frequency scanning systems that also utilise ALE operate as follows.
All stations in the communications network scan, and receive, frequencies throughout the High Frequency spectrum that they are allocated to use. Each station also transmits a “sounding” signal consisting of the stations address and a bit stream. This “sounding” signal is transmitted at random intervals and on a frequency also chosen at random. Other stations receive the “sounding” signal and record details based on the “sounding” signal. A Link Quality Analysis value is also allocated to the transmitting station based on the “sounding” signal. The details, and their corresponding Link Quality Analysis value, are converted to records in a database. The database represents stations that have been “heard” on particular frequencies, their signal quality and the time when the station was “heard”.
When one station wishes to call another station, the transmitting station searches its database for records on the receiving station. These records are then compared to the current time to determine the best frequency to use for establishing communication with the receiving station. The transmitting station and receiving station then communicate using the determined frequency.
The problems involved with this synchronisation method are:                A significant delay may be encountered at startup of the network as all stations in the communications network need to be registered on each station within the network's database. Thus, initial communication between stations in the network must involve “sounding” signals; and        The on-air “sounding” process uses network communication air-time and whilst in progress could impede normal voice communications for which the network is intended.        